Fan arrangements, in particular for computers, frequently have such a control unit in order to be able to control the rotational speed of the fan. A high fan rotational speed is associated with a high noise level, which many users find disturbing. Therefore, in high-quality computers, it is customary to adapt the rotational speed of the fan to the respective cooling requirement. In phases of low loading, a computer generates little heat and only a low cooling capacity is necessary, with the result that the fan can be operated with a low rotational speed.
The fan is controlled by the regulating transistor connected in series, which makes it possible to set the voltage present across the fan. The circuit principle employed is illustrated in FIG. 2, where a fan 1, a regulating transistor 2 and a current measuring resistor 3 are connected in series between terminals of an operating voltage UB. The current measuring resistor 3 has a very small value and may serve, in the case of electronically commutated fans, to detect the fan current and to determine the fan rotational speed therefrom on the basis of the commutation pulses contained therein.
The regulating transistor 2 makes it possible to influence the voltage across the fan 1 in a simple manner. A control unit 4 is provided for driving the regulating transistor 2. The voltage drop across the regulating transistor 2 and thus the voltage across the fan 1 change depending on the driving effected by the control unit 4. In the case illustrated, the regulating transistor 2 is operated as a voltage follower, that is to say the voltage at the emitter is defined by the voltage at the base and also the base-emitter voltage. This circuit is cited as representative of other circuit principles that can be realized in connection with a regulating transistor.
A concrete realization of a control unit from the prior art is shown in FIG. 3, where the control unit 4 comprises a control transistor 5 driven by an operational amplifier 6. A control input voltage Uin is applied to the inverting input of the operational amplifier 6. Depending on the magnitude of the control input voltage Uin, the control transistor 5 is turned on to a greater or lesser extent, thereby changing the voltage Ucontrol at a control output 7 of the control unit 4. The control input voltage Uin is generated by a rotational speed detection circuit 8 connected to the current measuring resistor 3. Such a rotational speed detection circuit 8 is disclosed in U.S. Pat. Nos. 4,097,789, 6,274,991 and WO 03/103129 A1. The content of each of these documents is hereby incorporated herein by reference.
In order to achieve a reliable rotational speed regulation of a fan 1, it is of great importance that the rotational speed detection circuit 8 can reliably identify the commutation pulses of the fan motor 1. What is problematic in the case of circuits according to the prior art, however, is that fluctuations may occur in the operating voltage UB, that is to say the operating voltage UB is composed of a DC voltage component UGL and an AC voltage component UAC. The AC voltage component UAC may have a similar frequency to the commutation operations, with the result that an erroneous identification of fan pulses may occur if said AC voltage component affects the current through the current measuring resistor. An abrupt fluctuation in the operating voltage will occur whenever other loads on the same power supply unit generate sudden load changes such as, by way of example, hard disks or processors with an activated power-saving function.
In order to solve this problem, it is known to provide a separate passive filtering of the operating voltage of the fan by means of inductor coils and capacitors. What is disadvantageous is that both components have to be given comparatively large dimensions and high additional costs thus arise.
In another solution to the problem, in order to identify commutations, current fluctuations of the fan are detected as a percentage and compared with a differential percentage which is greater than the percentage fluctuation of the operating voltage. In other words, if the operating voltage can fluctuate for example by ±5% and the identification threshold for a fan commutation is ±15%, for example, it can initially be assumed that no erroneous identification can occur.
In the case of this method, however, an erroneous identification of commutation pulses can nevertheless occur if the fan is operated with a reduced operating voltage. This can be seen upon consideration of the circuit according to the prior art in accordance with FIGS. 2 and 3, in which the regulating transistor 2 feeds a stabilized regulating voltage to the fan. While the fluctuating operating voltage is present at one terminal of the fan 1, a stabilized voltage is fed to the other terminal of the fan 1. Accordingly, the voltage across the fan 1 also fluctuates and a correspondingly fluctuating current flow through the fan 1 occurs. This fluctuating current is tapped off as a voltage across the current measuring resistor 3 and fed to the rotational speed detection circuit 8. This has the effect that a fluctuation in the operating voltage may lead to an erroneous identification. A commutation pulse is thus identified even though such a pulse is not present, but rather an interference voltage which has been caused for example by the activation of a processor in the power-saving mode.
The magnitude of the voltage fluctuations can be determined by means of the following calculation, a customary circuit in accordance with FIG. 3 being taken as a basis, in which the control voltage Ucontrol is generated by an N-MOS transistor in an open-drain configuration and an operational amplifier. The fan voltage results asUfan=UGL+UAC+UBE−Ucontrolin which case Ucontrol=Uin*(R1+R2)/R2 holds truewhere    UGL=DC voltage component of the operating voltage,    UAC=AC voltage component of the operating voltage,    UBE=base-emitter voltage of the regulating transistor 2, and    Ucontrol=stabilized regulating voltage.
Although the stabilized regulating voltage is variable for the rotational speed regulation of the fan, what is involved is a comparatively slow voltage change with a time constant typically lying in the region of 100 ms. By contrast, the time constant of commutation pulses is typically 1 ms.
If it is assumed in a computational example that UGL=12 V, UAC=±0.5 V, UBE=−0.7 V and Ucontrol=8 V, the following is obtained for the voltage across the fan
                              U          fan                =                                            12              ⁢                                                          ⁢              V                        ±                          0.5              ⁢                                                          ⁢              V                                -                      0.7            ⁢                                                  ⁢            V                    -                      8            ⁢                                                  ⁢            V                                                  =                              3.3            ⁢                                                  ⁢            V                    ±                      0.5            ⁢                                                  ⁢            V                                                  =                              3.3            ⁢                                                  ⁢            V                    ±                      15            ⁢                          %              .                                          
Despite an operating voltage fluctuation of just ±5%, the fan voltage accordingly fluctuates by ±15%, given an ohmic behavior of the fan, current fluctuations of ±15% would now be expected, so that the voltage at the current measuring resistor 3 also fluctuates by ±15%.
A further problem is posed by the nonlinear behavior of the internal fan electronics of commercially available fans. The switch-on/off threshold of such a typical fan is at an operating voltage of 3 V, that is to say that the current fluctuations may be much more than ±15%, specifically if, in the worst case, the fan continually switches on and off. The rotational speed detection circuit 8 then incorrectly interprets this switching on and off as a current fluctuation which has been caused by a commutation of the fan.